The present invention relates in general to a photographic film cassette, and more particularly to an improvement in a photographic film cassette of indicating on a cassette shell the degree of exposure of a photographic film.
There are known 35 mm film cassettes which have a film leader that does not protrude from a cassette shell, but which are easily loaded in a camera, as disclosed in, for example, U.S. Pat. Nos. 4,832,275, 4,834,306 and 4,846,418. The rotation of a spool by a film advancing mechanism in an unwinding direction causes the film leader of the photographic film to advance outside of the cassette shell through a film passage mouth of the film cassette.
In conventional devices, double exposure of photographic film can occur because there is no visual indication of whether a film cassette contains exposed or unexposed film. To overcome this problem, a film cassette as disclosed in Japanese Utility Model Laid-Open Publication No. 1-135441, provides a counter lever which is shiftable according to a change of the diameter of the film roll, and a counter for indicating a film feeding amount according to a detected diameter of the film roll, which detects a decrease in the diameter of the film roll around the spool as the photographic film is drawn out of the cassette shell.
In the film cassette as above, however, to detect the diameter of the film roll, the counter lever is in constant contact with the surface of the roll of film, which can scratch the surface of the photographic film. Irregularities, such as scratched surfaces, can cause the roll of film to loosen and disproportionately increase the diameter of the roll of film. As a result, a change in the film's diameter will not constantly correctly correspond to the amount that the photographic film is drawn from the film cassette, so that the indication by the counter is incorrect.
In some situations, the photographic film may contain frames which are exposed and frames which are c unexposed. In such situations, the roll is then rewound in the cassette shell, and the film cassette is unloaded from the camera. To later use the unexposed frames, the photographic film is reloaded in the camera, and advanced to wind past the exposed frames by referring to the indication in the counter. Because the indication is inaccurate, the photographic film can be advanced an excessive amount so that some unexposed portion of the photographic film might be inadvertently left unused. On the other hand, the film may not be advanced enough so that some portion overlapping the exposed frames might be double exposed.